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PYRIDINE

333

..4

with a thermometer with its bulb central. The flask is heated over the
aperture in an asbestos card in such a manner that 5 c.c. distil per minute •
when the temperature reaches 140°, the heating is interrupted, the liquid
that comes over being still collected and the              „.„„,.

volume of the distillate measured. The distilla-
tion is then continued up to 160° and the
volume of the distillate measured.

5.  Behaviour towards Water.—50 c.c. of
the pyridine are treated with 100 c.c. of water
to ascertain if the two liquids mix completely
and if the  solution  is  clear or more or less
opalescent.    The opacity may be   determined
best by looking through the mixture in a tube
such  as  is   indicated  under   i   (above)   and
determining   the   possibility   or  otherwise of
reading print of definite dimensions.

6.  Behaviour towards Caustic Soda.—In a graduated cylinder with
a ground stopper, 20 c.c. of the pyridine are shaken with 20 c.c. of caustic
soda solution, D 1-4 (50 grams of sodium hydroxide in 100 c.c.) and left
for an hour ; the volume of the upper layer, consisting of the pyridine bases,
is then read.   The difference between this volume and 20 c.c. gives the
water content.

7.  Determination of the Bases.—10 c.c. of the pyridine are diluted
with water to 100 c.c. and 10 c.c. of this solution titrated with normal sul-
phuric acid until a drop of the liquid produces on Congo red paper1 a blue
ring, which quickly disappears.    The result is expressed by indicating the
number of c.c. of normal acid employed.

A more exact determination is obtained by Francois' gravimetric
method,2 based on the insolubility in anhydrous ether of the additive com-
pound of pyridine hydrochlori.de with gold chloride. About o-i gram of
the material is weighed into a porcelain basin and treated with water, 20-30
drops of hydrochloric acid and excess of gold chloride solution, a precipitate
being formed and the liquid remaining deep yellow. The whole is evaporated
to dryness on a water-bath and, when all the hydrochloric acid is eliminated,
washed by decantation with anhydrous ether as long as any colour is still
removed (to eliminate the excess of gold chloride). The precipitate remain-
ing in the dish is calcined and the metallic gold weighed: Au X 0-401 =
pyridine, when pure pyridine is taken. With crude pyridine, the weight
of the gold must be multiplied by the coefficient 0-5583 (deduced from
the mean molecular weight of the bases, no).

*
* *

The requirements for pyridine bases to be used for denaturation are:
Test i :  should remain colourless or less coloured than the solution of iodine
indicated.

1 Prepared by immersing chemically pure filter-paper in i % aqueous Congo red
solution and allowing to dry.

z Comptes rendus, 1903, Vol. 137, p. 329.ong and fittedn